1. Field Of The Invention
The present invention relates to integrated circuit technology. More particularly, the present invention relates to user-programmable, normally-open antifuse structures which may be permanently altered to form a low resistance ohmic connection by the application of a relatively low programming voltage.
2. The Prior Art
Integrated electronic circuits are usually made with all internal connections set during the manufacturing process. However, because of high development costs, long lead times, and high manufacturing tooling costs of such circuits, users often desire circuits which can be configured or programmed in the field. Such circuits are called programmable circuits and they usually contain programmable links. Programmable links are electrical interconnects which are either broken or created at selected electronic nodes by the user after the integrated device has been fabricated and packaged in order to activate or deactivate respectfully the selected electronic nodes
Programmable links have been used extensively in programmable read only memory devices (PROMs). Probably the most common form of programmable link is a fusible link. When a user receives a PROM device from a manufacturer, it usually consists of an X-Y matrix or lattice of conductors or semiconductors. At each cross-over point of the lattice a conducting link, called a fusible link, connects X and Y lines. The PROM is programmed by blowing the fusible links at selected cross-over points to create an open circuit. The combination of blown and unblown links represents a digital bit pattern of ones and zeros signifying data which the user wishes to store in the PROM.
Such fusible link PROM systems present certain disadvantages. For instance, because of the nature of the conducting material in the link, relatively high voltage and high current levels are needed during programming to guarantee the complete blowing of the fusible links. Since the link is usually conductive, it needs large amounts of power dissipation to blow it. Also, the shape and size of the fusible link must be precise so that the link will function effectively as a conductor if it is not blown and will be a completely open circuit if it is blown by passing a predetermined amount of current through it. Therefore, very critical photolithographic steps and controlled etch techniques are required during the manufacturing process of fusible link PROMs. Finally, a large gap must be blown in the link in order to prevent it from later becoming closed through the accumulation of the conducting material near the blown gap. Fusible link memory cells are relatively large in order to accommodate the link and its associated selection transistor and, therefore, fusible link PROMs have high manufacturing costs because they take up large amounts of semiconductor area.
In recent years, a second type of programmable links, called antifuse links, have been developed for use in integrated-circuit applications. Instead of the thermal fusing programming mechanism causing an open circuit as is the case with fusible links, the programming mechanism in antifuse circuits creates a short circuit or relatively low resistance link. Antifuse links consist of two conductor and/or semiconductor materials having a dielectric or insulating material between them. During programming, the dielectric in between the conductive materials is broken down by applying a predetermined voltage across the conductive materials, thereby electrically connecting the conducting or semiconducting materials together.
Various materials have been suggested for both the conducting layers and the dielectric or insulating layer. Some of the suggested dielectric materials require a relatively high current and voltage during programming, require complex manufacturing techniques, and have low reliability during programming because it is difficult to control the reproduceability of the antifuse in its conductive state due to the complex nature of the programming mechanism involved
Some of the proposed dielectric insulators are doped amorphous silicon, polycrystalline silicon, oxides, titanates of transition metals, oxides of silicon, aluminum oxide and cadmium sulfide. The problems with approaches utilizing these materials, have been related to the need of a high current and voltage to program or the difficulty to repeatably manufacture and control their characteristics in both the on and off states. Materials such as cadmium sulfide, aluminum oxide and titanate present complicated technological problems because they are difficult to integrate into standard semiconductor processing.
Examples of known antifuse elements are found in the prior art using various insulating materials. Reference is made to: U.S. Pat. No. 3,423,646 which uses aluminum oxide, cadmium sulfide; U.S. Pat. No. 3,634,929 which uses single film of Al.sub.2 O.sub.3, SiO.sub.2, and Si.sub.3 N.sub.4 ; U.S. Pat. No. 4,322,822 which uses SiO.sub.2, U.S. Pat. No. 4,488,262 which uses oxides or titanate of a transition metal; U.S. Pat. No. 4,499,557 which uses doped amorphous silicon alloy; U.S. Pat. No. 4,502,208 which uses SiO.sub.2 ; U.S. Pat. No. 4,507,757 which uses SiO.sub.2 ; U.S. Pat. No. 4,543,594 which uses SiO.sub.2.
Most of the above patents either describe complicated technologies or need high breakdown voltages and currents, and/or are difficult to manufacture or do not meet the reliability requirements of state-of-the-art integrated circuits in both the on and off states.
The programming voltage required to create the low resistance path between the two electrodes is determined by the breakdown voltage of the dielectric. This voltage is a strong function of the dielectric thickness.
In order to construct a reliable antifuse device. The dielectric thickness must be chosen such that it will not break down by application of the voltages present on the device during the normal operation of the product. In addition, the thickness of the dielectric must be such that the capacitance of the normally-open unblown antifuse is low enough such that unblown antifuses does not significantly degrade speed power product performances of the device in which they are embodied.
These two requirements usually lead to the formation of dielectrics for antifuses which are relatively thick, therefore requiring high programming voltage, or/are otherwise complicated, as evidenced by the prior art antifuses referred to above. These prior art antifuses suffer from a more complicated fabrication process, lower circuit density, and/or higher programming voltages.
U.S. Pat. No. 4,823,181 discloses the use of a silicon dioxide-silicon nitride-silicon dioxide sandwich as a composite dielectric layer. The antifuse disclosed therein avoids many of the problems associated with other prior art antifuse structures. There is, however, still room for improvement of antifuse structures.
Mechanisms other than simple dielectric breakdown have been proposed and used for antifuse structures. U.S. Pat. No. 4,562,639 to McElroy, discloses a thin-oxide avalanche-breakdown antifuse element which may be programmed at a lower voltage than the oxide breakdown voltages. The programming mechanism utilized by the device disclosed in U.S. Pat. No. 4,562,639 is avalanche breakdown of the drain junction. The programming voltages utilized are on the order of 20 volts and the dielectric used is silicon dioxide. The device is used to select redundant rows or columns in a memory array or as a PROM device. In either case, the scheme disclosed requires the use of a select transistor in series with the antifuse element.